Two novel actinobacterial strains, designated C9-5T and C3-43, were isolated from soil samples of a cave in Jeju Island, Republic of Korea, and subjected to taxonomic study by a polyphasic approach. The organisms exhibited a typical rod-coccus developmental cycle during growth and grew at 10-30 °C, pH 5-9 and 0-3 % (w/v) NaCl. In 92 single-copy core gene sequence analysis, strain C9-5T was loosely associated with Rhodococcus tukisamuensis, albeit sharing low 16S rRNA gene sequence similarity (97.4 %). A combination of morphological and chemotaxonomic characteristics supported assignment with the genus Rhodococcus. With respect to 16S rRNA gene sequence similarity, the novel isolates showed the highest identity to the type strain of Rhodococcus subtropicus (98.7 % sequence similarity), followed by Rhodococcus olei (98.5 %) and Rhodococcus pedocola (98.4 %).The average nucleotide identity and digital DNA-DNA hybridization values between strain C9-5T and members of the genus Rhodococcus were ≤81.5 and ≤37.1 %, respectively. A set of physiological and chemotaxonomic properties together with overall genomic relatedness differentiated the novel isolates from members of the genus Rhodococcus, for which the name Rhodococcus spelaei sp. nov. is proposed. The type strain is C9-5T (=KACC 19822T=DSM 107558T). Based on genome analysis performed here, it is also proposed that Rhodococcus biphenylivorans Su et al. 2015 is a later heterotypic synonym of Rhodococcus pyridinivorans Yoon et al. 2000, Rhodococcus qingshengii Xu et al. 2007 and Rhodococcus baikonurensis Li et al. 2004 are later heterotypic synonyms of Rhodococcus erythropolis (Gray and Thornton 1928) Goodfellow and Alderson 1979 (Approved Lists 1980), and Rhodococcus percolatus Briglia et al. 1996 and Rhodococcus imtechensis Ghosh et al. 2006 are later heterotypic synonyms of Rhodococcus opacus Klatte et al. 1995.
Keywords: International Code of Nomenclature of Bacteria; Rhdococcus spelaei; Rule 42; later heterotypic synonym; natural cave.